Flat panel display devices have become a major area of interest to electronic related industries. Flat displays will provide more compact desktop and transportable computers and aid in the development of computer-telephones. The projected U.S. and worldwide markets for flat panel displays are estimated to reach 450 million and 2 billion dollars by the end of this decade, J. A. Castellano, Digital Design, May 1984, p. 2.
There are several approaches that are currently being examined and marketed. Among these are: flat cathode ray tubes, light emitting diodes, plasma displays, vacuum flourescent displays, liquid crystals, and other electroluminescent displays, J. I. Pankove, Topics in Applied Physics: Display Devices, Volume 40, J. I. Pankove (ed.), Springer-Verlag, New York (1980), p. 1. Liquid crystal displays are currently the most widely used for flat passive displays. The area of largest anticipated growth lies in devices which have a high contrast, bright background passive display with low power consumption. Such devices could effectively compete with and/or replace liquid crystal displays. Electrochromic devices are acknowledged as theoretically capable of producing the desired properties.
Electrochromism describes the induction of a color change in a medium as a result of charge transfer or electron transfer caused by an externally applied potential. The color changes are indications of induced chemical changes in the species of interest. For most chemical species exhibiting this effect, the change is from one color to another. As an example, viologen dye molecules change from yellow-orange to blue when reduced at a cathode. J. Bruinik, C. G. A. Kregting, and J. J. Ponjee, J. Electrochem. Soc. 124, 1853 (1977). Solid films of WO.sub.3 also show electrochromism with transparent films becoming blue upon reduction.
In order for electrochromic materials to be useful for display purposes, they must have optical absorption in the visible spectrum and exhibit mixed conduction capability (i.e. electronic and ionic). It is also highly desirable to exhibit high contrast from the background in order to modulate ambient light. Electrochromic materials generally have these properties. Electrochromic materials are usually operated with low voltages and can provide suitable contrasts with charge transfer of only several millicoulombs of electrical charge per square centimeter of display area. Erasure is easily made by polarity changes. These materials may also have the ability to hold images for the required response time of the human eye (about 0.1 second) and this further may allow for the use of memory effects. A major disadvantage of electrochromic displays is the lifetime of the device. Chemical degradation frequently occurs as usage time increases.
The most studied systems which utilize the electrochromic effect are displays based on WO.sub.3. B. W. Faughnan, Topics in Applied Physics: Display Devices, Volume 40, J. I. Pankove (ed.,), Springer-Verlag, New York, (1980), p. 181. Amorphous films of WO.sub.3 have high ion mobilities as necessary and exhibit coloring and bleaching between blue and transparent colors. The device lifetime is extremely sensitive to the presence of oxygen and water. To date, a commercial viable system based on the oxide films has yet to be produced.
Organic species have also been examined as an alternative but frequently lack the desired contrast since they convert between two distinct colors and do not have a transparent form. J. Bruinick, C. G. A. Kregting, and J. J. Ponjee, J. Electrochem. Soc. 124, 1853 (1977). M. M. Nicholson and F. A. Pizzarello, J. Electrochem. Soc. 127, 821 (1980).
Polyaniline is the chemical name given to the product of anodic oxidation of aniline. The formation of polymeric compounds by oxidation of aniline has been known for some time. S. Venkataraman, Chemistry of Synthetic Dyes, Volume II, Academic Press, New York (1952), p. 772. The products are highly colored films or solids. The first modern electrochemical study of this oxidation at solid electrodes was carried out by Adams and co-workers. D. M. Mohilner, R. N. Adams, and W. J. Argersinger, Jr., J. Am. Chem. Soc. 84, 3618 (1962). A polymeric product was obtained which they suggested to be an octamer of head to tail para coupling of aniline monomers. ##STR1## This octamer was prepared in sulfuric acid electrolyte. It has been suggested to be emeraldine sulfate, a highly colored salt which had been observed in previous studies involving chemical oxidation. Since their original paper, Adams and coworkers have acknowledged that other coupling modes (i.e. head-to-head or tail-to-tail) are possible. J. Bacon and R. N. Adams, J. Am. Chem. Soc. 90, 6596 (1968). Although the polymeric nature of this oxidation product has been suggested for many years, the full characterization has remained inconclusive. Renewed interest in the structural nature of the polymer has been generated by recent findings of its good electrical conductivity. A. F. Diaz and J. A. Logan, J. Electroanal. Chem. 111, 111 (1980). They noted that the polymer is conducting in both anodic and cathodic regions. They also noted that the film color can be altered by varying the electrode potential.
A more recent article has presented a brief spectral characterization of films grown on indium oxide electrodes. T. Kobayashi, H. Yaneyama, and H. Tamura, J. Electroanal. Chem. 161, 419 (1984).
Our invention is in part based on the fact that polyaniline films are conducting and although it has not been established, it is believed that there are ionic and electronic contributions to its conductive properties. It is expected that relatively high ion mobilities (particularly proton) are found for this films. The films are prepared in aqueous solution and do not dissolve. They are also relatively stable toward oxygen.
Our invention embodies a electronic display element useful in electronic color display devices. Broadly, our invention comprises two electrodes, at least one electrode being transparent, having electrolyte disposed therebetween. A thin film of polymeric aniline or its chemical derivatives is placed in electrical communication with at least one of said electrodes. In a preferred embodiment, the polymeric film is coated electrolytically on the anode using an acidic solution containing the monomeric aniline. After the polymer film is coated, the solution is replaced by an acidic electrolyte solution which does not contain aniline monomer. Applying different voltages across the interface between the polymer film and the electrolyte results in color changes of the film. Color changes achieved to date include blue, green, yellow and transparent. The color changes are sharp and distinct and repeated cycling of the voltage does not cause degradation of the film and the response time of the color change is short.
The display element of our invention overcomes the prior art problems of longevity and the prior art problems of the inability of the films to repeatedly produce color changes, including transparent, which are necessary for successful application of electrochromism in electronic color display devices. Further advantages of our invention are a display screen in a thin plate or rollable sheet which consumes a minimal amount of electrical power. Further, the area of the display device can be very large in reference to the physical limitations imposed on the presently available cathode-ray tubes. Most importantly, a multicolor display is achieved which capability is not available in present liquid crystal display devices.